using OrdinaryDiffEqLowStorageRK
using Trixi

# Up to version 0.13.0, `max_abs_speed_naive` was used as the default wave speed estimate of
# `const flux_lax_friedrichs = FluxLaxFriedrichs(), i.e., `FluxLaxFriedrichs(max_abs_speed = max_abs_speed_naive)`.
# In the `StepsizeCallback`, though, the less diffusive `max_abs_speeds` is employed which is consistent with `max_abs_speed`.
# Thus, we exchanged in PR#2458 the default wave speed used in the LLF flux to `max_abs_speed`.
# To ensure that every example still runs we specify explicitly `FluxLaxFriedrichs(max_abs_speed_naive)`.
# We remark, however, that the now default `max_abs_speed` is in general recommended due to compliance with the
# `StepsizeCallback` (CFL-Condition) and less diffusion.
dg = DGMulti(polydeg = 3, element_type = Quad(), approximation_type = SBP(),
             surface_integral = SurfaceIntegralWeakForm(FluxLaxFriedrichs(max_abs_speed_naive)),
             volume_integral = VolumeIntegralFluxDifferencing(flux_ranocha))

equations = CompressibleEulerEquations2D(1.4)

"""
    initial_condition_kelvin_helmholtz_instability(x, t, equations::CompressibleEulerEquations2D)

A version of the classical Kelvin-Helmholtz instability based on
- Andrés M. Rueda-Ramírez, Gregor J. Gassner (2021)
  A Subcell Finite Volume Positivity-Preserving Limiter for DGSEM Discretizations
  of the Euler Equations
  [arXiv: 2102.06017](https://arxiv.org/abs/2102.06017)
"""
function initial_condition_kelvin_helmholtz_instability(x, t,
                                                        equations::CompressibleEulerEquations2D)
    # change discontinuity to tanh
    # typical resolution 128^2, 256^2
    # domain size is [-1,+1]^2
    slope = 15
    amplitude = 0.02
    B = tanh(slope * x[2] + 7.5) - tanh(slope * x[2] - 7.5)
    rho = 0.5 + 0.75 * B
    v1 = 0.5 * (B - 1)
    v2 = 0.1 * sin(2 * pi * x[1])
    p = 1.0
    return prim2cons(SVector(rho, v1, v2, p), equations)
end
initial_condition = initial_condition_kelvin_helmholtz_instability

cells_per_dimension = (32, 32)
mesh = DGMultiMesh(dg, cells_per_dimension; periodicity = true)

semi = SemidiscretizationHyperbolic(mesh, equations, initial_condition, dg;
                                    boundary_conditions = boundary_condition_periodic)

tspan = (0.0, 1.0)
ode = semidiscretize(semi, tspan)

summary_callback = SummaryCallback()
alive_callback = AliveCallback(alive_interval = 10)
analysis_interval = 100
analysis_callback = AnalysisCallback(semi, interval = analysis_interval)
save_solution = SaveSolutionCallback(interval = analysis_interval,
                                     solution_variables = cons2prim)
callbacks = CallbackSet(summary_callback,
                        analysis_callback,
                        alive_callback, save_solution)

###############################################################################
# run the simulation

sol = solve(ode, CarpenterKennedy2N54(williamson_condition = false);
            dt = estimate_dt(mesh, dg), ode_default_options()..., callback = callbacks);
